The invention provides a process and apparatus to upgrade low-temperature energy recovered in thermal-separation processes to high-temperature energy which can be reused in the separation. It is known that fluid pairs can be used to upgrade energy quality through absorption. Ordinarily, this is accomplished in a closed-loop system wherein a lower-boiling component is fractionated from a higher-boiling component, vaporized around the distillation temperature of the higher-boiling component, and recontacted in a higher-pressure absorber to generate heat at a higher temperature which is used in the distillation process. This process also has been disclosed for an open-loop system using components which are the subject of the separation process, but the recombination during absorption requires recycle of the absorbed components to separation with resulting inefficiencies.